The present invention relates to electroacoustic transducers, i.e. loud speakers which are adapted to receive an electrical signal and to convert the same into sound energy for propagation. In particular the present invention is related to a form of loud speaker which is particularly adapted to reproduction of low and mid frequencies, i.e. frequencies of the range of 20 to 5,000 Hz.
Heretofore, loud speakers particularly those adapted for frequency work in the range from about 20 Hz to 5,000 Hz, have been limited in their effectiveness and faithfulness of reproduction due to one very distinct factor which is the slow propagating velocity of the acoustic information across the diaphragm. The sound propagates as transverse sound which is slow in comparison with longitudinal sound propagation. The diaphragms brake up into a multiplicity of resonant modes. When excited these resonances store elastokinetic energy causing a delay in sound emission till equilibrium is reached. When the excitation stops the stored elasto-kinetic energy is spent gradually re-emitted into unwanted overhanging sound waves, i.e. a failure of adequate transient response.
In typical cone type loudspeakers the diaphragm of the loudspeaker is capable of sound propagation by transverse waves in more than one direction, the radial direction and the circumferential direction. The sound propagation in the circumferential direction leads to a bell type vibrational modes, whereas propagation in a radial direction ultimately leads to a cone breakup. Each of these types of vibration are capable of causing interference with each other. Each of the types of vibrations tend to cause the cone to be less pliable for propagation of the other type of vibration in such a way that the increased rigidity involved causes changes in resonant frequency and therefore causes a resultant pitch variation in the modes being propagated. In other words, the wave which is being propagated causes an increase or possible decrease in the rigidity of the cone material and this causes the pitch shift either up or down correspondingly. The foregoing effects in relation in shifts and frequency variations and also in shifts in resonances result in gliding tones to which the ear is extremely sensitive due to speech recognition patterns which are highly developed in the human ear. In order to enhance the transient response of such cone diaphragm loud speakers, various damping materials have been utilized, such as viscous material diaphragm supports and massive quantities of sound deadening load. These materials in themselves absorb sound energy and therefor inherently make such conventional speakers inefficient. In addition, control of speaker motion has been obtained not only by the sound deadening loads into which they are operated but also by employing amplifiers having high damping factors and the like.
This effect causes frequency variations in existing resonances as function of time and resulting gliding tones to which our ear is extremely sensitive due to its use in speech recognition. The rigidity of a diaphragm is not constant but varies considerably when the diaphragm is engaged in the usual vibrational mode in which the waves propagate in a transverse mode; i.e. perpendicular to the diaphragm surface. The waves proposed for use in the present invention will not be propagating in a flat diaphragm but in a preformed, especially shaped, corrugated sheet which has greater rigidity and therefore greater sound propagating velocity and a higher resonant frequency. Such special shape enable the diaphragms of the present loud speaker to avoid internal resonances which cause such distortions.
In U.S. Pat. No. 3,636,278 entitled ACOUSTIC TRANSDUCER WITH A DIAPHRAGM FORMING A PLURALITY OF ADJACENT NARROW AIR SPACES OPEN ONLY AT ONE SIDE WITH THE OPEN SIDES OF ADJACENT AIR SPACES ALTERNATINGLY FACING IN OPPOSITE DIRECTIONS in the name of Oskar Heil, issued Jan. 19, 1972, a speaker system is disclosed in which its function can be characterized as providing a large moving diaphragm area to air motional area, sometimes called an air motion transformer. This type of speaker has been very successful in faithfully reproducing high frequencies in accordance with the principles set forth in said patent. However, the principles there disclosed suffer certain disadvantages when an attempt is made to apply those principles to the production of lower frequency sound, particularly in the forementioned range of 20 to 5,000 Hz. In general, the motional transformer as disclosed in U.S. Pat. No. 3,636,278 can be constructed for use at low frequencies but is found to be inefficient due to the high mass loading caused by air moving out the slots with consequent loss in transient response, particularly in the frequency range of high aural discrimination. As mentioned, the mass loading of the air itself is high because it is related to the velocity of the amount of air which must be moved. As is known, gross motion of a loud speaker at lower frequencies and the amount of physical air moved is much larger than at high frequencies. Since the motional transformer of said U.S. Pat. No. 3,636,278 is of the order of 5 to 1 that is to say, the air velocity is in range of 5 time greater than the velocity of the diaphragm materials causing such motion, and because the kinetic energy stored therein is proportional to the square of the air mass moved, (i.e. at factor of 25) it is seen that considerable additional kinetic energy must be exchanged between the air mass being moved and the speaker.
Accordingly, there is a need for a new and improved loudspeaker structure not subject to the foregoing limitations and disadvantages.